1. Technical Field
This invention relates to calibration standards for surface inspection systems.
2. Background Art
The need for the calibration of wafer surface inspection systems has been widely recognized by the semiconductor manufacturing industry. These systems are used for the detection, identification, and measurement of the number and sizes of particles on, or in, the surface of semiconductor wafers. The prior absence of a suitable calibration standard has resulted in skepticism regarding the daily performance and reliability of these instruments. In the prior art, there have been several attempts to fabricate a standard using a variety of known features to provide a light-scattering response that is comparable to the levels observed with particulate contamination. These features include the use of polystyrene (latex) spheres, etched pits/holes, raised frustums, and thin wires or fibers.
One of the most prevalent methods of establishing the sensitivity of these systems has been the atomization and deposition of a dilute suspension of monodisperse, polystyrene spheres in a random pattern upon the surface of a semiconductor wafer. Despite the availability, uniformity, and repeatability of these spheres, there are a number of problems associated with this method of calibration. The major limitation is that each sphere must be characterized as to its size and its location on the wafer using optical and/or electron microscopy. This long and tedious procedure is required to distinguish each sphere from either surface contamination, or from clusters of spheres that have agglomerated. The spacing between each sphere must also be considered, because if the spacing between two or more spheres is less than the resolution capabilities of the detector, then the spheres will not be resolved from one another and will be detected as a single, larger sphere. A further limitation is that when the wafer becomes heavily contaminated, it must be discarded because of the difficulty in distinguishing the spheres from the surface contamination.
Due to the numerous problems with using latex spheres, there have been several attempts to fabricate a substrate with an easily recognizable pattern of either etched pits (or holes) or raised frustums to simulate particulate contamination. Although these standards are permanent and capable of being cleaned of any surface contamination, there are some inherent disadvantage associated with there use. First, unless these features are circular in shape, their scattering responses will change with the orientation of the substrate within the system. Even if the features are circular, the response is still dependent upon the illumination and detection geometries of the instrument, since these features scatter light in an anisotropic manner. Therefore, these prior calibration standards have failed to have universal application, and can only serve as references for maintaining the repeatability of a specific type of wafer surface inspection system.
It is highly desirable to have a universal calibration standard which can be used not only as a reference for maintaining the repeatability of a specific type of wafer surface inspection system, but also to have universal application such that different types of wafer surface inspection systems can be readily compared and/or set at a common sensitivity.